1. Field of Invention
The present invention relates to a high-speed motor such as a polygon mirror, a hard disc and, in particular, relates to a vibration attenuation technique for a motor in which a dynamic pressure bearing is used as a radial bearing.
2. Related Art
Of various kinds of motors, the motor disclosed in Japanese Patent Unexamined Publication No. Sho. 64-3318, for example, employs an air dynamic pressure bearing as a radial bearing so that a rotor is floated to the thrust direction by utilizing magnetic force generated between a magnet disposed on a rotor side and a magnet disposed on a stator side. Thus, since the rotor is in a completely non-contact state during the rotation thereof, the rotor can be rotated at a high speed. In such an arrangement, an annular air chamber for an air damper is formed between a fixed shaft and the rotor in a manner that the air chamber is communicated with the outside through a small hole (orifice). According to such an arrangement, the motor can be provided with an air damper by the small hole and the annular air chamber for an air damper, so that the vertical vibration of the rotor can be suppressed due to the resistance of the air when passing through the small hole.
Further, Japanese Patent Unexamined Publication No. Hei. A-8-196056 discloses a motor in which the effect similar to that of the aforesaid publication is obtained by forming grooves extending from an annular air chamber for an air damper to the outside with respect to a dynamic pressure portion in stead of a small hole forming the air damper.
In order to constitute such an air damper, a small hole or grooves serving as an orifice is required. However, in the case of forming such a hole by the drill processing like the prior art, the minimum diameter .o slashed. and length of the hole are limited to about 0.4 mm and 5 mm, respectively, even if it is desired to made them smaller. Further, in the case of combining a member having been subjected to such a hole making process with another member by means of a screw etc. thereby to form an annular air chamber for an air damper, there arises a case that the annular air chamber for an air damper communicates with the outside through the combined surface and the chamber does not serve as a damper. Furthermore, as the motor is further miniaturized, it is required to make the resistance of the flow path of the orifice of the air damper larger. Thus, although a narrow long hole is required as the orifice, it is difficult to form such a small and narrow hole by the normal boring process.
In the case of forming the groove at the dynamic pressure generating portion, there arises a problem that the rigidity of a dynamic pressure bearing is degraded.